1. Field of the Invention
The present invention relates to a contact-type image sensor which has photoelectric conversion elements for converting light reflected from a document to electric signals corresponding to an image formed on the document, and more particularly, to a structure which reduces the leakage of unnecessary light into the photoelectric conversion elements.
2. Description of the Related Art
A contact-type image sensor comprises a light source, an erect-image focusing optical system, and a line sensor. The light source applies light to a document. The optical system, such as a rod-lens array, receives the light reflected from the document and focuses an erect image on the light-receiving surface of the line sensor, with a magnification of one. The line sensor has a linear array of photoelectric conversion elements, which have a length substantially equal to the width of the document. The photoelectric conversion elements convert the image focused by the optical system, to electric signals. In other words, the line sensor generates electric signals (image signals) which correspond to the image formed on the document.
The rod lens array comprises a number of rod lenses juxtaposed in the same direction as the photoelectric conversion elements, with their optical axes aligned parallel to each other. Each of the rod lenses has two focal points located at the same distance from the midpoint on the axis of the lens. The document is placed in the plane containing the first focal point of every rod lens, whereas each photoelectric conversion element is located at the second focal point of the corresponding rod lens. Hence, the document opposes the photoelectric conversion elements, with the rod lens array interposed between the document, on the one hand, and the conversion elements, on the other hand.
FIG. 7A illustrates the positional relationship among each rod lens 71, the document 72, and each photoelectric conversion element 73. As is evident from FIG. 7A, the light reflected from a given point P1 on the document 72 (i.e., the point corresponding to one pixel) is focused by the rod lens 71 on the light-receiving surface of the conversion element 73, which is located at point P2 opposing the point P1.
If rod lens 71 has ideal characteristics, the light focused at point P2 should be exclusively the light reflected from the point P1 on the document 72. Referring to FIG. 7B, the light A reflected from a point P3 spaced part from the point P1 should be focused at a point P4 which is symmetrical to the point P3 as is indicated by the arrow B, after passing through the rod lens 71.
However, the rod lens can hardly have such ideal characteristics, due to the limited manufacturing precision. A stray light is inevitably generated in the lens 71 as any light passes through the lens 71. Consequently, there is the possibility that the light reflected from any point (including the point P3) near the point P1, as well as the light reflected from the point P3, may be applied to the light-receiving surface of the photoelectric conversion element 73. Thus, the conversion element 73 generates a pixel signal which is superposed with the components corresponding to the stray lights resulting from the lights reflected from points other than the point P1.
This problems will be described in greater detail, with reference to FIGS. 8A, 8B, and 9.
FIG. 8A illustrates the case where a white line on a black background is to be read. As shown in the figure, the point P1 is on the white line so that the white line may be read. However, the light reflected from any point within a circle S, the center of which is the point P1, are focused on the photoelectric conversion element 73 because of the characteristics of the rod lens 71. Hence, the lights reflected from those parts S1 and S2 of the black background, which are within the circle S, are also applied through the lens 71 to the photoelectric conversion element 73. The element 73 therefore receives less light than in the case where no dark parts of an image are located within the circle S, as is illustrated in FIG. 8B. As a result, as is shown in FIG. 9, the voltage OA output by the element 73 is lower than the voltage OB the element 73 should generate if no dark parts of an image were located within the circle S.
Thus, although the point P1 is placed on the white line to read the line, the voltage output by the transducer element 73 changes in accordance with the condition of those parts of the image which are located near the point P1. As a consequence, the pixel signal output by the element 73 is one influenced by the parts of the image which are near the reading position.